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From A History of Mathematical Notations by Florian Cajori, 1929
. . . Enström . . . inclines to the view that the predominance [in Descartes] of x over y and z is due to typographical reason, type for x being more plentiful because of the more frequent occurrence of te latter x, to y and z in the French and Latin languages.8There is nothing to support the hypothesis on the origin of x due to Wertheim,
Wertheim, Gustav, 1843-1902. Title Anfangsgr�nde der Zahlenlehre / von Gustav Wertheim. Publisher Braunschweig : F. Viewig, 1902. Description xii, 427 p. , [1] leaf of plates : port. ; 23 cm. Language German
Diophantus, of Alexandria. Title Die Arithmetik und die Schrift �ber Polygonalzahlen des Diophantus von Alexandria. �bersetzt und mit anmerkungen begleitet von G. Wertheim. Publisher Leipzig, B. G. Teubner, 1890. Description ix, 346 p. 4 il. 23 cm. Language German
Wertheim, Gustav, 1843-1902. Title Elemente der Zahlentheorie, von Gustav Wertheim. Publisher Leipzig : B. G. Teubner, 1887. Description ix, 381, [1] p. 23 cm. Language German
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Wertheim, Gustav. [ ? ? ] Title Die Arithmetik des Elia Misrachi : ein Beitrag zur Geschichte der Mathematik / von Gustav Wertheim. Edition 2. verb. aufl. Publisher Braunschweig : Friedrich Vieweg, 1896. Description 68 p. ; 23 cm. Language German
http://www.columbia.edu/cu/libraries/inside/projects/ebooks/prd/testing/Elia/html/pages/FID3.htmlQuestion why is this one listed separately by the University of California and no dates are given ?